The best price found from United States to Vietnam is on March 19th, 2015, travelling with EVA Air. See more options, compare prices and book: search flights.
There are four established international airports in Vietnam. Others including Phu Quoc and Hue are officially classified as 'international' but at the time of research had no overseas connections (apart from the odd charter).
It’s hard to get reservations for flights to/from Vietnam during holidays, especially Tet, which falls between late January and mid-February.